THe nice thing about Li polymer batteries is they can be custom sized to fit the maximum juice in the space the mfr has available. The bad part is that often means the mfr is the only source for those batteries, and if they have declared them unreplaceable they likely won’t sell you one.
I usually see cell phone and laptop batteries go pretty limp after a couple of years. Lucky for me I guess my cell has broken a few times and the warrantee replacements have always come with a new battery. My daughter’s phone (same brand & battery, different model) has held up well, but was pretty much useless off the charger after 2 years. New battery perked it right up.
My Neuros player (not a polymer battery) is around 7 years old and down to about 15% of the original play time. Luckily, it’s a standard lithium battery, replaceable at about $10. Not a priority so far as I usually use it on a lighter adapter.
